In recognition of their inherently cleaner and potentially more efficient fuel burning properties, fluidized bed boilers are now seriously being considered as viable supplements to the traditional pulverized coal and stoker fired vapor generating units of today.
Briefly, a fluidized bed boiler burns granulated coal in a floating fluid-like suspension called a fluidized bed. In addition to the coal, a sorbent (usually limestone) is introduced into the bed to absorb the noxious gases generated as a result of the burning process. By introducing fluidizing air from beneath the burning zone, the burning coal actually floats on a mobile cushion of air as it is consumed. By allowing the coal to float in suspension, fuel combustion is improved. As a result of the enhanced combustion process, greater quantities of heat may be produced. And, as a consequence of the introduction of the sorbent, undesirable pollution levels are substantially reduced.
During normal operations, it is expected that a fluidized bed boiler will experience large fluctuations in the load impressed upon it. During peak demand periods, the boiler will be called on to deliver increased amounts of steam. Conversely, during slack demand periods, decreased amounts of steam will be required.
It has been suggested that one possible method of steam quantity and/or steam temperature control may be to raise or lower the height of the bed. By altering the depth of the bed, the amount of steam generator tube surface submerged within the bed may be varied as well. However, by lowering the bed height, the newly exposed tubes disposed above the bed may have a deleterious effect upon the performance of the boiler. The exposed tubes may quench the gases leaving the bed and limit above bed burning. This will result in an increase in both unburned combustibles and pollution emissions.
Another frequently mentioned method of steam output control is to modulate the temperature of the bed itself. This method of control may be accomplished by either placing additional bed compartments into service or removing them from service (slumping) as conditions dictate. Furthermore, the quantity of fuel introduced into the bed may be regulated as well.
Unfortunately, all of the above enumerated methods present difficulties. It has been determined that to maintain acceptable combustion efficiency and emission levels, it may be necessary to operate the bed within a narrow temperature range irrespective of the load impressed upon the boiler. Thus, fluctuations in bed temperature and bed height may be undesirable.
Clearly, an alternate means for controlling steam output is necessary.